When an electrical arc occurs inside an oil-filled tank, part of the oil vaporizes and an enormous pressure builds up within fractions of a second. The pressure relief device is the line of defense that prevents the tank from turning into an exploding vessel.
The Primary Function
The Pressure Relief Device (PRD) is a mechanical device mounted on the surface of the tank that opens automatically when the internal pressure exceeds a pre-set limit, rapidly venting pressurized oil and gases to the outside and protecting the tank from deformation or rupture. Some designs are equipped with a counter and contact points that send an alarm or trip signal when the device operates — the practical reference notes that some systems are set to signal once the pressure exceeds roughly 30% above the set value.
The Difference Between It and the Buchholz Relay
| Buchholz Relay | Pressure Relief Device | |
|---|---|---|
| Responds to | Accumulated gases and oil movement in the pipe | A sudden rise in pressure inside the tank |
| Speed of event | Gradual and major faults | Instantaneous, violent faults |
| Action | Alarm or electrical trip command | Immediate mechanical pressure release + signal |
Field Inspection Points
- Verify the device's integrity and check for any oil traces around it — oil traces mean it has operated before or is leaking.
- Operation of the device is a major event: the transformer must not be returned to service before a thorough investigation (DGA, insulation, internal inspection if needed).
- Test its signal circuit as part of periodic protection testing.
Sample answer: The pressure relief device protects the transformer tank from rupture: it opens automatically when there is a sudden rise in internal pressure (resulting from an internal arc that vaporizes the oil) and instantly releases the pressure, while sending an alarm or trip signal. The difference from the Buchholz relay is that Buchholz detects accumulated gases and oil movement in the conservator tank pipe and suits gradual faults, while the PRD responds mechanically to instantaneous pressure in violent faults.
Returning the transformer to service after the pressure relief device has operated simply by resetting the device to its position. The device's operation indicates a violent event inside the tank, and operating the transformer before a full investigation is a gamble with the transformer and site safety.
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